If a UK intelligence agency’s holiday puzzle wasn’t enough to keep you entertained, how about something a little more difficult? This crossword puzzle is based on the computer science language of regular expressions, and it should keep you busy for... some time.

Regular expressions are a sequence of characters that define a search pattern. They’re what are normally used to define processes like “search and replace” that most of us take for granted. But if you’re nerdy enough you can actually write them from scratch. Or, uh, use them to solve this crossword puzzle.

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You can fill in an interactive version of the puzzle, that was put together by Greg Grothaus, here. Good luck!